Michael B. Schmithorst, Subramanian Prasad, Ahmad Moini, Bradley F. Chmelka, “Direct evidence of paired framework heteroatoms in a zeolite catalyst and their significance for catalytic reactivity,” Journal of the American Chemical Society, 145, 18215–18220 (2023).
The significance of this work is that it provided, for the first time, direct and unambiguous evidence of closely paired aluminum heteroatoms in the frameworks of industrially relevant zeolite catalysts. Aluminum (or other) heteroatoms in nanoporous, crystalline, aluminosilicate zeolite frameworks impart local negative charges, which are balanced by exchangeable cations that impart diverse adsorption, separation, and catalytic properties. Measuring, understanding, and controlling aluminum distributions in zeolite frameworks were long sought goals, especially with respect to the formation of paired heteroatom sites. Such paired Al sites desirably alter the types and distributions of exchangeable cations, in particular catalytically important H+ acid sites or divalent Cu2+ cations, which influence transition states that have dramatic effects on the rates and selectivities of hydrocarbon conversion reactions or the reduction of NOx compounds. However, paired (or other) aluminum configurations were previously exceedingly challenging to detect and quantify, due to their nonstoichiometric and nonperiodic distributions within zeolite frameworks. In contrast, powerful 2D solid-state 27Al–29Si and 29Si–29Si NMR analyses (Fig. 4A) overcame such challenges by being sensitive to the local environments of covalently bonded 27Al–O–29Si and 29Si–O–29Si moieties, respectively, enabling different types of paired framework aluminum atoms to be detected directly and unambiguously. This was demonstrated specifically for two H+-chabazite zeolite catalysts with similar bulk Al contents that were indistinguishable by conventional analytical methods, but which exhibited significant differences in the rates of methanol dehydration (Fig. 4B), an industrially important reaction, that was shown to be due to differences in the types and populations of paired Al sites. Manjesh Kumar, Zachariah J. Berkson, R. John Clark, Yufeng Shen, Nathan A. Prisco, Zhiyuan Zeng, Haimei Zheng, Lynne B. McCusker, Jeremy C. Palmer, Bradley F. Chmelka,* Jeffrey D. Rimer,* “Ultrathin mordenite crystals synthesized using cooperative organic structure-directing agents,” Journal of the American Chemical Society, 141, 20155–20165 (2019).
The significance of this paper is that it reported novel synthesis and understanding of an important zeolite catalyst with very small (<100 nm) crystallite dimensions that reduced mass transfer limitations and significantly improved catalyst performance. The paper focused on the zeolite mordenite, a commercial catalyst that is used in diverse processes, such as dehydration of alcohols to olefins, oxidation of methane to methanol, (hydro)isomerization, cracking, alkylation, and carbonylation. Mordenite has one-dimensional sub-nanometer pores that present mass transport limitations to the diffusion of reactants and product species, which leads to rapid deactivation by coking, the effects of which are exacerbated by large crystallites.
We used two types of organic structure-directing agents (OSDAs), which enabled the crystallization of heterogeneous zeolite catalysts that significantly outperformed materials generated by conventional syntheses that rely on a single OSDA. Atomic-scale 2D NMR 27Al–29Si, 27Al{1H}, and 13C{1H} NMR correlation analyses (Fig. 5) established the proximities and interactions of occluded OSDAs with distinct zeolite framework moieties, which account for their important synergistic effects on mordenite crystallization. This was the first time that two OSDAs acting in tandem were used to generate 100-nm zeolite crystallites, which resulted in important benefits to their particular reduced effects of cooking
